A partial denture is composed of an artificial tooth which substitutes for a missing tooth, a denture base to which the artificial tooth is fixed and which makes close contact with a residual ridge, a clasp which is fixed to the denture base and is removably fitted to a remaining tooth, a rest formed integrally with the clasp, and the like. The following three functions are required for the partial denture.
(A1) Supporting function against the occlusal pressure;
(A2) Retaining function against the separation force; and
(A3) Grasping function against the horizontal force applied to the denture.
The rest generally has the supporting (grasping) function, and more specifically it is considered to function as follows: (r1) to transmit the occlusal pressure applied to the partial denture to a clasped tooth (i.e., a remaining tooth that is restrained by the clasp for supporting the partial denture; also called an “anchor tooth”); (r2) to prevent sinkage of the partial denture; (r3) to hold the clasp in place; and (r4) to suppress rocking of the partial denture.
Meanwhile, the clasp is considered to prevent separation or movement of the denture base by restraining the clasped tooth with a hooked arm. More specifically, it is considered that the clasp is required to exert the following functions: (c1) to prevent separation of the partial denture by a hook portion of the arm that extends from the top of the tooth crown to the undercut of the clasped tooth to act against the separation (lifting) force; and (c2) to prevent rotation of the partial denture with an indirect retaining device as the fulcrum. With this function to prevent rotation, the clasp serves as the indirect retaining device to suppress movement of the partial denture.
The rest and clasp described above are often formed as a metal article that is cast in one piece. Alternatively, they may be formed with wire. Such a common, conventional clasp is described in Non-Patent Documents 1 and 2.    Non-Patent Document 1: “Revised New Edition, Osborne: Partial Dentures”, Ishiyaku Publishers, Inc., July 1977, p. 166.    Non-Patent Document 2: Minoru Ai, “Design Manual for Partial Dentures According to Cases”, Gakken Shoin, p. 36.